Natural killer T (NKT) cells recognize CD1d presented lipid antigens. NKT cells have been strongly implicated in host defense to microbial infection due to their ability to rapidly secrete large amounts of cytokines and their influence on the activation of NK cells, macrophages, dendritic cells, B cells and T cells. Except for the striking evidence that aGalCer can pharmacologically activate NKT cells, little is known about the antigens that activate NKT cells physiologically in vivo. Most evidence suggests that CD1d molecules present self-lipid antigens for the development and activation of NKT cells. In mice, self-antigens for Va14 invariant NKT cells are acquired by CD1d molecules that traffic to lysosomes and loading is dependent on saposins. In contrast, the self-antigens for diverse TCR NKT cells may be acquired in the ER. These features are different for human Vot24 invariant NKT cells, as they are stimulated by CD1d molecules that do not traffic through lysosomes and are not saposin dependent. It is assumed that self-antigens are made by CD1d+ ARC and thus are considered endogenous. However, we have now determined that CD1d presented self-lipids also are acquired from extracellular sources where lipids are mainly bound to iipoproteins such as apoE. Thus, we separate endogenous and exogenous pathways of CD1d self-antigen presentation. Here, we propose to identify the self-lipid antigens that bind to CD1d along the secretory route or when trafficking through endosomal compartments and determine which are stimulatory for NKT cells (Aim 1). Because several lines of evidence suggest that important differences exist, we will compare the CD1d selfantigens and their ability to stimulate NKT cells in mice with those in humans (Aim 2). We will then determine the self-antigens acquired by the newly appreciated apoE dependent exogenous pathway (Aim 3). Since lipid and lipoprotein metabolism are changed profoundly in the acute phase reaction during infection, we will determine how endogenously and exogenously acquired CD1d presented self-lipids are altered in inflammation (Aim 4). These studies seek to address the fundamental mechanisms and unanswered questions regarding the self-antigens that activate CD Id-restricted NKT. This has significance for understanding the function of NKT cells and is important for host defense to infection and other diseases.